Catalysts comprising from 54.2 to 56.3 mass percent of calcium oxide, from 42.8 to 45.7 mass percent of phosphorus pentoxide, the balance (up to 100%) being water (see, for example, "Kinetics and Catalysis", 1960, volume 1, issue 2, pp. 247-256) are widely used at the present time.
In reactions of decomposition of 1,3-dioxanes, the catalysts of the above-mentioned composition possess a low activity resulting in a low space velocity of the raw material feed equal to 0.5 to 0.7 h.sup.-1 and a high operating temperature of the catalyst that is equal to approximately 370.degree. to 390.degree. C.
The activity of catalysts is known to depend upon the acidity thereof which is determined by the number and efficiency of the active centres and can be characterized by the degree of decomposition of 1,3-dioxanes under similar operating conditions.
The degree of decomposition of 1,3-dioxanes is defined as the ratio of the amount of 1,3-dioxanes converted, to that of 1,3-dioxanes used, both being expressed in moles. The level of decomposition is expressed in percent.
The selectivity of the above-mentioned catalysts is also not high and lies at a level of 68 to 75 mole %.
The selectivity is defined as the ratio of the amounts in moles of isoprene formed to the amount in moles of 1,3-dioxane converted, expressed in percent.
Selectivity is quantitatively dependent upon catalyst composition and structure, as well as upon the process conditions under which the catalyst operates.
Increased selectivity will lead to reduced stockfeed consumption rates per unit of finished product, thus per ton of isoprene. A relatively low selectivity of the catalyst would result in high feedstock consumption rates in the isoprene production, varying between ca. 2.10 and 2.25 kg of 1,3-dioxanes per 1 kg of isoprene.
Furthermore, the above-mentioned catalysts feature a higher coke deposition ability of 3.0 to 5.0 mole %. Coke deposition is defined as the ratio of the amount in moles of coke deposited to the amount in moles of 1,3-dioxane converted, expressed in percent. Increased coke deposition leads to a decrease in the selectivity of the 1,3-dioxane decomposition process and increases the unproductive waste of energy used for the catalyst recovery process that involves the burning-out of the deposited coke by a steam-air mixture at a temperature of 400.degree. to 500.degree. C.
Also known in the prior art are catalysts comprising 12.8 mass percent of cadmium oxide, 40.1 mass percent of calcium oxide, 40.0 mass percent of phosphorus pentoxide, and water up to 100 mass percent (see, for example, "Izvestia visshich utchebnich zavedenij", Chimija i chimitcheskaja technologija, vol. 7, No. 5, pp. 801-805).
The catalysts of the above composition show a low activity and a comparatively short life of 300 h.
The catalyst life depends upon many factors including catalyst composition and structure, catalyst activity, operating temperature, and coke deposition.
Furthermore, the catalysts of said type show a low selectivity of 78 to 80 mole %. The known catalysts feature also a low efficiency within 0.20 to 0.25 t/h of isoprene per cubic meter of the catalyst. The efficiency of the catalyst depends upon its activity and selectivity, as well as upon the space velocity of raw material feed.
In spite of the advantages of the known catalysts, no commerical process has been so far developed for the decomposition of 1,3-dioxanes with the use of such catalysts, since there is no catalyst as yet devised which would exhibit the selectivity and stability such as to justify a commercial process with a high yield of the final product.